The invention relates to an alarm system capable of detecting when an article is removed from a predetermined position.
In known alarm systems used to deter or detect shop-lifters for example, each article on an open display has been interconnected in a loop by means of a wire coupled to an alarm. Such a system is obtrusive and tends to detract from the display. Adding or removing articles with such a display requires disconnecting the interconnected wires in the loop which can become tedious.
An alternative known system makes use of pressure pads which are connected to an alarm and removal of an article triggers this alarm. In such a system it is necessary to have a pressure pad for each article on display if removal of one is to be detected and thus changes in the number of articles displayed requires addition or removal of one or more pads and changes in the system connections. Thus such a system does not lend itself to frequent changes in the display.
It has also been proposed to use the capacitive effect of displayed articles to trigger an alarm by a system which uses a metal sheet as a conductor connected to an alarm system and each displayed article is placed on this sheet which overlies a shelf. In such a system effectively it was necessary to measure the capacitance of the articles between this electrode and earth and such a system has been shown to be unreliable due to stray capacitance effects and such a system could instigate an alarm condition merely by a customer being in the vicinity of the display which proves to be an embarrassment to both the customer and the shop-keeper.
The present invention is concerned with a system which overcomes the above drawbacks to provide a reliable, flexible and typically unobtrusive system which can cope with a number of articles together if required.